U.S. patent application number 10/977241 was filed with the patent office on 2006-05-04 for led package with front surface heat extractor.
Invention is credited to Catherine A. Leatherdale, Andrew J. Ouderkirk, John A. Wheatley.
Application Number | 20060091414 10/977241 |
Document ID | / |
Family ID | 36190597 |
Filed Date | 2006-05-04 |
United States Patent
Application |
20060091414 |
Kind Code |
A1 |
Ouderkirk; Andrew J. ; et
al. |
May 4, 2006 |
LED package with front surface heat extractor
Abstract
Light sources are disclosed utilizing LED dies having at least
one emitting surface. An optical element is disclosed for
efficiently extracting light out of an LED die by controlling the
angular distribution of the emitted light. The optical element has
an input surface that is optically coupled to the emitting surface,
an output surface that is larger in surface area than the input
surface, and at least one intermediate surface. A heat sink
thermally coupled to the intermediate surface of the optical
element extracts heat from the emitting surface of the LED die via
the optical element.
Inventors: |
Ouderkirk; Andrew J.;
(Woodbury, MN) ; Wheatley; John A.; (Lake Elmo,
MN) ; Leatherdale; Catherine A.; (St. Paul,
MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Family ID: |
36190597 |
Appl. No.: |
10/977241 |
Filed: |
October 29, 2004 |
Current U.S.
Class: |
257/99 ;
257/E33.073 |
Current CPC
Class: |
H01L 33/58 20130101;
H01L 33/60 20130101; H01L 33/64 20130101 |
Class at
Publication: |
257/099 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Claims
1. A light source, comprising: an LED die having at least one
emitting surface; an optical element including an input surface
optically coupled to the at least one emitting surface, an output
surface larger than the input surface, and at least one
intermediate surface; and a heat sink thermally coupled to the at
least one intermediate surface.
2. The light source of claim 1, wherein the optical element is an
optical collimator.
3. The light source of claim 1, wherein the optical element is
shaped as a taper.
4. The light source of claim 1, wherein the optical element is
shaped as a batwing.
5. The light source of claim 1, wherein the input surface of the
optical element contacts the at least one emitting surface of the
LED die.
6. The light source of claim 1, wherein the input surface of the
optical element is bonded to the at least one emitting surface of
the LED die.
7. The light source of claim 1, wherein a refractive index of the
optical element is within 25% of a refractive index of the emitting
surface.
8. The light source of claim 1, wherein a refractive index of the
optical element is at least 1.8.
9. The light source of claim 1, wherein a refractive index of the
optical element is at least 2.0.
10. The light source of claim 1, wherein the at least one
intermediate surface is highly reflective.
11. The light source of claim 10, wherein the optical element
comprises a reflective metallic coating over the at least one
intermediate surface.
12. The light source of claim 10, wherein the optical element
comprises a low index coating over the at least one intermediate
surface.
13. The light source of claim 1, further comprising a thin
optically conducting layer disposed between the input surface and
the emitting surface.
14. The light source of claim 13, wherein the thin optically
conducting layer comprises a liquid.
15. The light source of claim 13, wherein the thin optically
conducting layer is also thermally conducting.
16. The light source of claim 1, wherein the optical element
comprises a material having a thermal diffusivity of at least about
0.01 cm.sup.2/s.
17. The light source of claim 1, further comprising a second heat
sink thermally coupled to a base of the LED die.
18. A light source, comprising: an LED die having a primary
emitting surface; a transparent optical element including an input
surface optically coupled to the primary emitting surface, an
output surface larger than said input surface, and at least one
intermediate surface; and means for extracting heat out of the at
least one intermediate surface.
19. The light source of claim 18, wherein the optical element is an
optical collimator.
20. The light source of claim 18, wherein the optical element is
shaped as a taper.
21. The light source of claim 18, wherein the optical element is
shaped as a batwing.
22. The light source of claim 18, wherein the input surface of the
optical element contacts the primary emitting surface of the light
source.
23. The light source of claim 18, wherein the input surface of the
optical element is bonded to the primary emitting surface of the
light source.
24. The light source of claim 18, wherein a refractive index of the
optical element is within 25% of a refractive index of the primary
emitting surface.
25. The light source of claim 18, wherein a refractive index of the
optical element is at least 1.8.
26. The light source of claim 18, wherein a refractive index of the
optical element is at least 2.0.
27. The light source of claim 18, wherein the at least one
intermediate surface is highly reflective.
28. The light source of claim 27, wherein the optical element
comprises a reflective metallic coating over the at least one
intermediate surface.
29. The light source of claim 27, wherein the optical element
comprises a low index coating over the at least one intermediate
surface.
30. The light source of claim 18, further comprising a thin
optically conducting layer disposed between the input surface and
the primary emitting surface.
31. The light source of claim 30, wherein the thin optically
conducting layer comprises a liquid.
32. The light source of claim 30, wherein the thin optically
conducting layer is also thermally conducting.
33. The light source of claim 18, wherein the optical element
comprises a material having a thermal diffusivity of at least about
0.01 cm.sup.2/s.
34. The light source of claim 18, further comprising a heat sink
thermally coupled to a base of the LED die.
35. A light source, comprising: an LED die having a primary
emitting surface, a transparent optical element including an input
surface optically coupled to the proximate the primary emitting
surface, an output surface larger than said input surface, and at
least one side surface disposed between the input surface and the
output surface; and means for extracting heat out of the at least
one side surface.
36. The light source of claim 35, wherein the optical element is an
optical collimator.
37. The light source of claim 35, wherein the optical element is
shaped as a taper.
38. The light source of claim 35, wherein the input surface of the
optical element contacts the primary emitting surface of the LED
die.
39. The light source of claim 35, wherein the input surface of the
optical element is bonded to the primary emitting surface of the
LED die.
40. The light source of claim 35, wherein a refractive index of the
optical element is within 25% of a refractive index of the primary
emitting surface.
41. The light source of claim 35, wherein a refractive index of the
optical element is at least 1.8.
42. The light source of claim 35, wherein a refractive index of the
optical element is at least 2.0.
43. The light source of claim 35, wherein the at least one side
surface is highly reflective.
44. The light source of claim 43, wherein the optical element
comprises a reflective metallic coating over the at least one side
surface.
45. The light source of claim 43, wherein the optical element
comprises a low index coating over the at least one side
surface.
46. The light source of claim 35, further comprising a thin
optically conducting layer disposed between the input surface and
the primary emitting surface.
47. The light source of claim 46, wherein the thin optically
conducting layer comprises a liquid.
48. The light source of claim 46, wherein the thin optically
conducting layer is also thermally conducting.
49. The light source of claim 35, wherein the optical element
comprises a material having a thermal diffusivity of at least about
0.01 cm.sup.2/s.
50. The light source of claim 35, further comprising a heat sink
thermally coupled to a base of the LED die.
Description
RELATED PATENT APPLICATIONS
[0001] The following co-owned and pending U.S. patent application
is incorporated by reference: "LED PACKAGE WITH NON-BONDED OPTICAL
ELEMENT", Attorney Docket No. 60216US002, filed 29 Oct. 2004.
FIELD OF INVENTION
[0002] The present invention relates to light sources. More
particularly, the present invention relates to light sources in
which light emitted from a light emitting diode (LED) is extracted
using an optical element.
BACKGROUND
[0003] LEDs have the inherent potential to provide the brightness,
output, and operational lifetime that would compete with
conventional light sources. Unfortunately, LEDs produce light in
semiconductor materials, which have a high refractive index, thus
making it difficult to efficiently extract light from the LED
without substantially reducing brightness, or increasing the
apparent emitting area of the LED. Because of a large refractive
index mismatch between the semiconductor and air, an angle of an
escape cone for the semiconductor-air interface is relatively
small. Much of the light generated in the semiconductor is totally
internally reflected and cannot escape the semiconductor thus
reducing brightness.
[0004] Previous approaches of extracting light from LED dies have
used epoxy or silicone encapsulants, in various shapes, e.g. a
domed structure over the LED die or formed within a reflector cup
shaped around the LED die. Encapsulants typically have a higher
index of refraction than air, which reduces the total internal
reflection at the semiconductor-encapsulant interface thus
enhancing extraction efficiency. Even with encapsulants, however,
there still exists a refractive index mismatch between a
semiconductor die (typical index of refraction, n of 2.5 or higher)
and an epoxy encapsulant (typical n of 1.5).
[0005] FIG. 1 shows another approach for providing an LED with
improved light extraction efficiency (U.S. Patent Application
Publication No. US 2002/0030194A1) (Camras et al.). This approach
uses a transparent optical element 2 having a refractive index
greater than about 1.8, bonded to an LED die 4. A disadvantage of
this approach is that when the optical element 2 is bonded to the
LED die 4, the bonded system incurs stress forces from each element
expanding as it heats up during operation.
[0006] LEDs need to be operated at a relatively low junction
temperature, typically no more than 125 to 150.degree. C. This
limits the maximum current flow and, correspondingly, the output of
the LED. Poor heat management can also adversely impact LED
lifetime by causing the LED die to run hotter than desired at a
given current. Enhancing heat extraction from the LED die can
increase the driving current thus providing higher light intensity
and longer lifetime. Known methods of extracting or dissipating
heat from the LED die include extracting heat through the base of
the LED die (typically the side opposite the primary emitting
surface). Other methods include adding a heat dissipating fluidic
coolant to the LED package, for example as described in U.S. Pat.
No. 6,480,389 (Shie et al.).
SUMMARY
[0007] Although advancements have been made, LEDs still have
potential to be even brighter. It would be advantageous to have an
LED package that efficiently extracts heat from the light emitting
side of the LED die. The present application discloses light
sources that utilize LED dies having at least one emitting surface.
An optical element is disclosed having an input surface that is
optically coupled to the emitting surface, an output surface that
is larger in surface area than the input surface, and at least one
intermediate surface. A heat sink thermally coupled to the
intermediate surface of the optical element extracts heat from the
emitting surface of the LED die via the optical element.
[0008] The above summary of the present invention is not intended
to describe each disclosed embodiment or every implementation of
the present invention. The Figures and the detailed description
below more particularly exemplify illustrative embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, where like reference numerals designate like elements.
The appended drawings are intended to be illustrative examples and
are not intended to be limiting. Sizes of various elements in the
drawings are approximate and may not be to scale.
[0010] FIG. 1 is a schematic diagram of an optical collimator
bonded to a light emitting diode of a Prior Art system.
[0011] FIG. 2 is a schematic side view illustrating an optical
element and LED die configuration in one embodiment.
[0012] FIG. 2a is a close-up view of a portion of the schematic
side view shown in FIG. 2.
[0013] FIGS. 3a-3c are schematic views of exemplary shapes of the
optical element.
[0014] FIGS. 4a-4e are schematic side views of additional exemplary
shapes of the optical element.
[0015] FIG. 5a is a schematic cross section view of an optical
element used in some embodiments.
[0016] FIG. 5b is a cross sectional side view of a clamp fixture
used in some embodiments.
[0017] FIG. 5c is a cross sectional side view of an LED die mounted
on a circuit board used in some embodiments.
[0018] FIG. 6a is a top view of the optical element shown in FIG.
5a.
[0019] FIG. 6b is a top view of the clamp fixture shown in FIG.
5b.
[0020] FIG. 7 is a cross sectional view of an assembled LED package
in accordance with some embodiments.
DETAILED DESCRIPTION
[0021] As described in more detail below, the present system
provides a light source with an optical element for efficiently
extracting light out of an LED die by modifying the angular
distribution of light emitted by the LED die. The optical element
is optically coupled to the emitting surface an LED die to
efficiently extract light.
[0022] In some embodiments, the optical element is also thermally
coupled to the LED die to permit heat removal from the LED die. To
further extract heat away from the optical element, a thermally
coupled heat sink clamp is added.
[0023] In other embodiments, an LED die is optically coupled to the
optical element without use of any adhesives or other bonding
agents between the LED die and the optical element. This allows the
optical element and the LED die to move independently as they each
expand when heated during operation. Absence of a bond or
mechanical decoupling eliminates stress forces on the optical
element and the LED die that may be present in a system bonded with
adhesive or other bonding agents.
[0024] FIG. 2 is a schematic side view illustrating a configuration
of an optical element 20 and an LED die 10 in one embodiment of the
present system. The optical element 20 is transparent and
preferably has a relatively high refractive index. Suitable
materials for the optical element include without limitation high
index glasses (e.g. Schott glass type LASF35, available from Schott
North America, Inc., Elmsford, N.Y. under a trade name LASF35) and
ceramics (e.g. sapphire, zinc oxide, zirconia, diamond, and silicon
carbide). Sapphire, zinc oxide, diamond, and silicon carbide are
particularly useful since these materials also have a relatively
high thermal conductivity (0.2-5.0 W/cm K).
[0025] In one embodiment, the optical element 20 is shaped in the
form of a taper as shown in FIG. 2. A tapered optical element 20
can have numerous forms, including without limitation those shown
in FIGS. 3a, 3b, and 3c. The optical element can be in the form of
other shapes, such as those depicted in FIGS. 4a-e, as well as
other shapes not shown. The tapered optical element 20 shown in
FIG. 2 is a particularly advantageous shape of the optical element.
In FIG. 2, the tapered optical element 20 has an output surface 130
that is larger than an input surface 120. Tapered shapes, including
a truncated inverted pyramid (TIP) shown in FIG. 3a, a truncated
cone shown in FIG. 3b, and a shape with parabolic sidewalls as
shown in FIG. 3c, and combinations thereof, provide the additional
benefit of collimating light and are referred to herein as optical
collimators. Using an optical collimator to extract light out of an
LED die is particularly advantageous because it provides control
over the angular distribution of light emitted. Additional shapes
for optical collimators will be apparent to those skilled in the
art. For example, a TIP shape, shown in FIG. 3a can be modified to
have curved sidewalls similar to those shown in FIG. 3c. Other
variations are contemplated. In addition, as will be described
below, optical collimators can also be shaped such that the
direction of emitted light is changed by the optical element 20.
When made of high index materials such as those mentioned above,
such optical elements increase light extraction from the LED die
due to their high refractive index and collimate light due to their
shape, thus modifying the angular emission of light. It will be
understood by those skilled in the art that when collimation is
less important or is not desired other shapes of optical element 20
may be used.
[0026] The LED die 10 is depicted generically for simplicity, but
can include conventional design features as known in the art. For
example, LED die 10 can include distinct p- and n-doped
semiconductor layers, buffer layers, substrate layers, and
superstrate layers. A simple rectangular LED die arrangement is
shown, but other known configurations are also contemplated, e.g.,
angled side surfaces forming a truncated inverted pyramid LED die
shape. Electrical contacts to the LED die 10 are also not shown for
simplicity, but can be provided on any of the surfaces of the die
as is known. In exemplary embodiments the LED die has two contacts
both disposed at the bottom surface as shown in FIG. 5c. This LED
die design is known as a "flip chip". The present disclosure is not
intended to limit the shape of the optical element or the shape of
the LED die, but merely provides illustrative examples.
[0027] The optical element 20 shown in FIG. 2 has an input surface
120, an output surface 130, and at least one intermediate side
surface 140 disposed between the input surface 120 and the output
surface 130. If the optical element is shaped in the form of a TIP,
as shown in FIG. 3a, then such an optical element 203a contains
four intermediate side surfaces 140a. If the optical element is
rotationally symmetric, than it will have a single side surface.
For example if optical element 20 is shaped as an inverted cone as
shown in FIG. 3b or shaped with parabolic sidewalls as shown in
FIG. 3c, then such an optical element 203b or 203c, respectively,
has a single side surface 140b or 140c, respectively. Other shape
variations can be used. Each optical element depicted in FIGS. 3a-c
contains an input surface 120a-c and an output surface 130a-c,
respectively. The shapes and cross sections of the input surface
and the output surface can vary. Exemplary shapes are shown as
input surfaces 120a-c and output surfaces 130a-c. FIG. 3a shows a
square cross section, while FIGS. 3b-c show circular cross sections
of the output surface. Other cross sectional shapes are also
contemplated, e.g. an optical element having a square input surface
and a rectangular output surface. In the examples shown in FIGS.
3a-c, the output surface is shown to be flat and parallel to the
input surface. Other arrangements are also contemplated, for
example an output surface that is angled with respect to the input
surface. FIGS. 4a-c show additional embodiments, in which the shape
of the output surface is curved to control the angle of emitted
light.
[0028] In one embodiment, depicted in FIG. 4a, an optical element
204a is shown having a curved output surface 180a. The optical
element 204a is made from a single structure, for example cut from
a single block of material.
[0029] In another embodiment, shown in FIG. 4b, an optical element
204b having a curved output surface 180b is shown. The optical
element 204b is made by joining a tapered element 22 having a flat
output surface 182 to a lens element 24 having a flat input surface
184 and a curved output surface 180b. The tapered element 22 and
the lens element 24 can be made of either the same material or of
two or more different materials with similar optical properties,
and in some embodiments similar thermal properties, depending on
design considerations. The input surface 184 is adhered to or
otherwise joined to the output surface 182 using conventional
means. FIG. 4b shows a lens element 24 of the same cross sectional
size as the output surface 182 of the tapered element 22.
[0030] FIG. 4c shows another example of an optical element 204c,
including a lens element 24 that is larger in cross section than
tapered element 22. For example, the diameter of the lens element
24 could be twice the diameter of the output surface 182 of the
tapered element 22. In FIG. 4c, a phantom line shows the portion
where the tapered element 22 is joined with the lens element 24.
The portion of the input surface 184 that is outside the cross
sectional area of the output surface 182 is shown in FIG. 4c as
surface 180d. The shapes depicted in FIGS. 3a-c and 4a-c are
exemplary and it is understood that other variations of these
shapes can also be used. Preferably, the tapered element 22 has
equal or higher index of refraction than the lens element 24.
Additional description of compound optical elements can be found in
co-filed and co-owned U.S. patent application titled "HIGH
BRIGHTNESS LED PACKAGE WITH COMPOUND OPTICAL ELEMENT(S)", Attorney
Docket No. 60218US002, which is incorporated herein by reference in
its entirety.
[0031] FIGS. 4d-e depict additional embodiments of the present
system. In FIG. 4d, an optical element 204d is a wedge shape with
an output surface 180d positioned perpendicular to an input surface
120d. Two intermediate surfaces 190d are shown, a first
intermediate surface that is parallel to the output surface 180d
and a second intermediate surface that defines a plane angled to
join opposing edges of the first intermediate surface 190d and
output surface 180d. In this embodiment, light emitted by the LED
die 10 at the primary emitting surface 100 is redirected by about
90 degrees by optical element 204d at the second intermediate
surface 190d, as shown by light rays 210. The top view of a wedge
shaped output element could be rectangular, trapezoidal, pie
shaped, semicircular, or any combination thereof.
[0032] In another embodiment, shown in FIG. 4e, an optical element
204e is a "batwing" structure, having opposing output surfaces 180e
and an intermediate surface 190e formed into a "V" shape. A batwing
structure can have various cross sectional shapes including,
without limitation, square, rectangular, or circular
cross-sections. One example of such a batwing structure can be
thought of as a wedge shape of FIG. 4d that has been rotated around
a normal line 200 to form the batwing shape shown in FIG. 4e. If
the cross section is circular the output surfaces 180e form a
cylindrical shape. Light emitted by the emitting surface 100 in a
random pattern centered around a normal line 200 is redirected 90
degrees from normal 200 into a ring formed around the cylinder
defined by output surface 180e. Such a batwing structure takes a
Lambertian light distribution at its input surface and modifies it
to a torroidal light distribution centered around the normal. It is
noted that in shapes such as the batwing shape discussed above, the
surface referred to as the intermediate surface 190e can be
physically disposed at the top portion of the optical element,
rather than on a lateral portion as in some of the previous
embodiments. As described in previous embodiments, the output
element can be made of a single material or from several materials,
for example as shown joined at phantom line 192, shown in FIG. 4e.
These embodiments are particularly suited for use as backlights in
liquid crystal display (LCD) panels, where brightness uniformity
can be important. Whether made of a single material or of two
materials joined at phantom line 192, the distance between input
surface 120e and the surface defined by line 192 as well as the
angle of faceted surfaces 190e can be optimized for a specific
application.
[0033] In some embodiment the optical element and the LED die are
positioned close together to allow optical coupling without use of
additional optical materials. FIG. 2 shows a gap 150 between the
emitting surface 100 of the LED die 10 and the input surface 120 of
optical element 20. Typically, the gap 150 is an air gap and is
typically very small to promote frustrated total internal
reflection. Preferably, the thickness of the gap 150 is less than a
wavelength of light in air. In LEDs where multiple wavelengths of
light are used, the gap 150 is preferably at most the value of the
longest wavelength. One example of a suitable thickness for gap 150
is less than 200 Angstroms. Another example of a suitable thickness
for gap 150 is less than 50 Angstroms. In addition, it is preferred
that the gap 150 be substantially uniform over the area of contact
between the emitting surface 100 and the input surface 120, and
that the emitting surface 100 and the input surface 120 have a
roughness of less than 20 nm, preferably less than 5 nm. In such
configurations, a light ray emitted from LED die 10 outside the
escape cone or at an angle that would normally be totally
internally reflected at the LED die-air interface will instead be
transmitted into the optical element 20. To promote optical
coupling, the surface of input surface 120 can be shaped to match
the emitting surface 100. For example, if the emitting surface 100
of LED die 10 is flat, as shown in FIG. 2, the input surface 120 of
optical element 20 can also be flat. The size of input surface 120
may either be smaller, equal, or larger than LED die emitting
surface 100. Input surface 120 can be the same or different in
cross sectional shape than LED die 10. For example, the LED die can
have a square emitting surface while the optical element has a
circular input surface. Other variations will be apparent to those
skilled in the art.
[0034] In some embodiments of the present system, the optical
element is optically coupled to LED die without bonding. This
allows both the LED die and the optical element to be mechanically
decoupled and thus allows each of them to move independently. For
example, the optical element can move laterally with respect to LED
die 10. In another example both optical element and LED die are
free to expand as each component becomes heated during operation.
In such mechanically decoupled systems the majority of stress
forces, either sheer or normal, generated by expansion are not
transmitted from one component to another component. In other
words, movement of one component does not mechanically affect other
components. This configuration is particularly desirable where the
light emitting material is fragile, where there is a coefficient of
expansion mismatch between the LED die and the optical element, and
where the LED is being repeatedly turned on and off.
[0035] One example of an LED package with a mechanically decoupled
optical element is a system in which optical element 20 is in
optical contact with LED die 10 via gap 150 as shown in FIG. 2. In
such an example, gap 150 may be an air gap, small enough to promote
frustrated total internal reflection, as described above.
[0036] Another example of an LED package with a mechanically
decoupled or non-bonded optical element is a system in which
optical element 20 is optically coupled to LED die 10 via a thin
optically conducting layer 60, as shown in FIG. 2a. FIG. 2a is a
close-up view of a portion of the schematic side view shown in FIG.
2 but with a thin optically conducting layer 60 disposed within gap
150. Examples of materials suitable for the optically conducting
layer 60 include index matching oils, and other liquids or gels
with similar optical properties. Optionally, optically conducting
layer 60 is also thermally conducting. A thin layer of an index
matching oil, or other similar liquid or gel, can be used to
enhance light extraction into the optical element 20. Preferably
the liquid or gel has a refractive index greater than that of the
LED die 10 but less than the refractive index of the optical
element 20. The thickness of such a thin optically conducting layer
can be greater than the size of an air gap. In this embodiment LED
die 10 need not be optically close to optical element 20 because
optically conducting layer 60 acts to optically couple the LED die
10 to the optical element 20. The size of optically conducting
layer 60 is determined by the refractive index of the material
used. Preferably, the thin optically conducting layer 60 is of a
size and made from a material optimized to promote frustrated total
internal reflection at the LED die--optically conducting layer
interface thus extracting more light from the LED die into the
optical element. Preferably, the thickness of optically conducting
layer 60 is on the order of a wavelength of light in that
material.
[0037] Index matching oils or similar liquids or gels have an added
benefit of higher thermal conductivity, which helps extract heat,
as well as light, out of LED die 10 and into optical element 20. In
some embodiments, thin optically conducting layer 60 is also
thermally conducting.
[0038] FIGS. 5-7 depict another embodiment of the present system.
In the exemplary embodiment depicted in FIGS. 5-7, an optical
element has a TIP shape, shown in more detail in FIG. 3a. FIG. 5a
depicts a schematic cross section side view of an optical element
20. A low index coating 70 is added to each side surface 140 to
promote total internal reflection. The low index coating 70 has a
refractive index substantially lower than both the refractive index
of LED die 10 and the refractive index of optical element 20.
Preferably the refractive index of low index coating is less than
about 1.5, more preferably less than 1.4. Optionally, the optical
element 20 can be coated with a reflective material such as a metal
layer or interference reflector, or combinations thereof using
conventional methods.
[0039] FIG. 5b depicts a schematic side view of a clamp fixture 30
that may be added to the system shown in FIG. 2. When assembled as
shown in FIG. 7, the clamp fixture 30 serves to hold optical
element 20 in place while positioned directly over and aligned with
LED die 10. Clamp fixture 30 can be made of plastic, metal, or
other suitable material. In reference to FIG. 5a, clamp 30 has a
hollow opening 34 shaped to receive the optical element 20. Hollow
opening 34 is defined by side surface 142, which is formed to
conform to the shape of side surface 140 of optical element 20.
Clamp 30 has an additional cut away portion 36, defined by surface
144 and shaped to receive LED die 10 and optionally allow extra
space on any or all sides of the LED die 10. FIG. 5b also shows
fiducials 32 which serve to line up clamp fixture 30 with fiducials
42 on circuit board 40, shown in FIG. 5c, so that optical element
20 is positioned directly above LED die 10. FIG. 5c depicts a cross
sectional side view of an exemplary configuration of circuit board
40 and LED die 10. In this example, LED die 10 is a flip chip, with
contact leads 12 disposed opposite the top emitting surface 100. It
is to be understood that side surfaces of LED die 10 may also emit
light. The optional additional space of cut away portion 36 can be
filled with air to promote TIR at the side surfaces of LED die 10,
so that light can be recycled and have another chance to escape
from LED die 10 into optical element 20.
[0040] The elements shown in FIGS. 5a, 5b and 5c are assembled
together as described below to form a system shown in FIG. 7.
Optical element 20 is coated with low index coating 70 as shown in
FIG. 5a. In some embodiments clamp fixture 30 is metal and is
prepared by machining a solid metal block to the shape shown in
FIG. 5b. Optical element 20 is inserted into clamp 30 at hollow
opening 34. The sides 142 forming opening 34 are soldered to sides
140 of optical element 20, preferably using low melting point
solder. Clamp 30 and optical element 20 are then positioned over
circuit board 40 shown in FIG. 5c, matching fiducials 32 and 42, so
that optical element 20 can be centered directly over LED die 10.
The pieces are then soldered together at solder contact points 38
and 48. The assembled system combining elements of FIGS. 5a, 5b and
5c is shown in FIG. 7. Top views of clamp 30 and optical element 20
are depicted in FIGS. 6a and 6b, respectively.
[0041] In another embodiment, clamp fixture 30 also serves as a
heat sink. In this embodiment, clamp fixture 30 can be made from a
high thermal conductivity and thermal diffusivity material (e.g.
copper) and does not need to be optically transparent. Clamp 30 in
this embodiment further removes heat from the optical element 20
allowing the LED to be operated at higher driving currents thus
producing higher brightness. Typical thermal diffusivity values for
materials used for optical element 20 are: flint glass--0.004
cm.sup.2/s; Sapphire--0.11 cm.sup.2/s; Silicon carbide--more than
1.6 cm.sup.2/s. Typical thermal diffusivity for copper is 1.2
cm.sup.2/s.
[0042] Optionally, another heat sink 50 can be added, as shown in
FIG. 7. Heat sink 50 extracts heat from LED die 10 via the base of
the LED die (typically the side opposite the primary emitting
surface).
[0043] In embodiments using the clamp fixture 30 as a heat sink,
optical element 20 need not be optically close to LED die 10 and
can be bonded or non-bonded. For example, optical element 20 can be
bonded to LED die 10 using inorganic thin films, fusable glass frit
or other bonding agent. Preferably a bonding agent with high
thermal conductivity and a similar index of refraction is used to
maximize heat transfer and optical transmission. Alternatively,
optical element 20 can be held in place over LED die 10 using clamp
30 while optical and thermal coupling between optical element 20
and LED die 10 is achieved using a thermally conducting layer, e.g.
an index matching fluid, gel or adhesive with appreciable thermal
conductivity, as described above. Typical thermal conductivity for
a suitable index matching oil is about 0.01 W/cm K.
[0044] In embodiments where the clamp fixture 30 does not serve as
a heat sink, the optical element 20 is held over the LED die 10
using the clamp 30 while optical coupling is achieved either via
gap 150 or via optically conducting layer 60.
[0045] Optical elements disclosed herein can be manufactured by
conventional means or by using precision abrasive techniques
disclosed in co-filed and co-owned U.S. patent application titled
"PROCESS FOR MANUFACTURING OPTICAL AND SEMICONDUCTOR ELEMENTS",
Attorney Docket No. 60203US002, and U.S. patent application titled
"PROCESS FOR MANUFACTURING A LIGHT EMITTING ARRAY", Attorney Docket
No. 60204US002, both of which are incorporated herein by
reference.
[0046] While the invention is amenable to various modifications and
alternative forms, specifics thereof have been shown by way of
example in the drawings and will be described in detail. It should
be understood, however, that the intention is not to limit the
invention to the particular embodiments described. On the contrary,
the intention is to cover modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
* * * * *